Understanding Florida’s Red Tide

In this Florida Sea Grant file photo, fish killed by a previous red tide litter a beach in southwest Florida.

By Betty Staugler, Florida Sea Grant Agent with UF/IFAS Extension in Charlotte County

Update 10/2/2018 — The information presented below pertains to the red tide bloom on the southwest coast of Florida. Red tide is now also occurring along the Panhandle and Atlantic coast of Florida near Palm Beach County. Studies are underway to determine how the red tide has moved to these areas. For up-to-date information on red tide status, visit the Florida Fish and Wildlife Conservation Commission Red Tide Report page: http://myfwc.com/redtidestatus

This red tide bloom has been trying for everyone. It is hard to see the harsh impacts the bloom has had on our coast, wildlife, and economy. I have been fielding a number of questions about the bloom, its causes and when it might end. See below for answers to some of the most frequently asked questions.

What is red tide? Red tides are caused by a type of plantlike organism called a dinoflagellate, which also is ubiquitous in lakes, rivers, estuaries and the oceans. The particular dinoflagellate that causes Florida’s red tide blooms, Karenia brevis, can literally make water look blood red and occurs primarily in marine waters.

What caused this red tide bloom? While red tides can form around the world, the organism that causes Florida’s red tide, Karenia brevis, is found almost exclusively in the Gulf of Mexico from Mexico to Florida. Red tides have historically formed off Florida even before humans settled here. Scientists know that red tide blooms originate 10-40 miles offshore, and winds and currents bring them inshore. Red tide growth is fueled by nutrients, in particular nitrogen and phosphorus. The sources of these nutrients vary among the offshore, nearshore, and estuarine environment.

Was this red tide bloom caused by the fresh water released from Lake Okeechobee through the Caloosahatchee River? No. The current red tide bloom began in late 2017, long before the current rainy season and freshwater releases from Lake Okeechobee. See my related blog post for related information.

Is the blue-green algae in the Caloosahatchee River fueling the current red tide bloom? Possibly. Nutrients from the natural flow of fresh waters along the coast of Florida have always had a probable influence on the extent of red tide. This year is unique — both red tide and blue-green algae are blooming at the same time. This didn’t happen during the 2016 blue-green algae bloom. As a result, scientists currently do not know much about how or if the two blooms are interacting. These studies are currently underway.

Is it OK to eat shellfish or locally caught finfish at a restaurant or purchase them from a seafood market during an algae bloom?

During a red tide bloom – According to the Florida Fish and Wildlife Conservation Commission, store-bought and restaurant-served bivalves, such as clams and oysters, are safe to eat during a red tide bloom because the shellfish are monitored by the government for safety. Commercially available shellfish are often not locally harvested and, if harvested locally, are tested for red tide toxins before they are sold. However, recreational harvest of bivalves during a red tide bloom is unsafe and banned. Edible parts of other animals commonly referred to as shellfish such as crabs, shrimp and lobsters are not affected by the red tide organism and can be eaten. But, do not eat the hepatopancreas — also known as green stuff, tomalley or mustard. Consuming finfish is also safe if the fish are filleted before eaten. Although toxins may accumulate in the guts of fish, these areas are disposed of when the fish are filleted.

During a Microcystis bloom – Although there are no federally established guidelines, anytime toxic cyanobacteria are in the water, consumption of seafood from affected and nearby areas should be avoided. Microcystis toxins can accumulate in the edible tissue of fish and shellfish, and cooking or boiling does not reduce toxins. The level of toxicity is dependent on the Microcystis species involved and the degree of bloom. The highest toxin concentrations occur in internal organs, such as in the liver and kidneys of fish, and hepatopancreas of crustaceans such as blue crab. Toxin levels will decrease over time when animals are no longer exposed to them.

Why has this red tide bloom been so bad? A number of factors are likely at play this year. For instance, higher than normal estuary and nearshore nutrient runoff could be contributing to the intensity and duration of this bloom. Residents and tourists may also be aware of the Saharan dust deposits that have been blown into the Gulf of Mexico over the last few months. Iron-rich Saharan dust not only gives us brilliant sunrises and sunsets, it also stimulates the bloom of another blue-green algae called Trichodesmium. Trichodesmium has the ability to obtain, or “fix” nitrogen from the atmosphere and convert it to ammonium, where it is then available to red tide cells. Ongoing onshore winds and currents have also kept the red tide inshore much longer than usual.

Wait a minute – doesn’t blue-green algae only occur in fresh water? Most blue-green algae does occur in fresh water, but there are also marine and estuarine species, like Trichodesmium. It’s important to note that not all blue-green algae is toxic and for those that are, the level of toxicity is largely dependent on the specific toxin produced. Trichodesmium does produce toxins, but researchers have not documented any negative effects of Trichodesmium on marine life or people in Florida. In fact, only a few specialized organisms are able to consume Trichodesmium.

Did Hurricane Irma play a role in this year’s red tide bloom? Scientists observed turbid waters from nearshore to approximately 30-40 miles offshore for several months after Hurricane Irma. However they don’t specifically know what role this played in the present bloom, if any.

Is this the longest red tide bloom on record? No. The longest red tide bloom since the Florida Fish and Wildlife Conservation Commission began collecting data in 1953 lasted 30 months, between 1994 and 1997. Six other red tide events have lasted as long as or longer than our current bloom. Keep in mind of course that this bloom is not yet over.

Is this the worst red tide bloom on record? I guess that depends on how we define “worst.” This is definitely a significant red tide event with serious impacts to fish, marine mammals, sea turtles, residents and coastal businesses. These types of impacts are typical during severe red tide events. However, more fish have died during this red tide in Lee and Charlotte counties compared to past severe red tide events, and it’s not over. In 2005, a red tide bloom resulted in a 2000 square mile dead zone in the Gulf of Mexico that extended from New Port Richey south to Sarasota. We haven’t seen that yet, but it could be our future. And, in 2015, three red tide blooms occurred simultaneously — one along the coast of Texas, another in the northern Gulf of Mexico from Louisiana through the Florida Panhandle, and a third in southwest Florida. Each of these could be described as unprecedented.

What can we expect as our climate changes? Warmer water due to climate change is predicted to cause algae to bloom more often, more intensely, and in more water bodies. Due to the synergistic relationship between increasing water temperature and nutrients, climate change will also make it harder to control blooms once they’re initiated. It is imperative that we reduce nutrient inputs to our lakes, rivers, estuaries and coastal ocean waters today.

When will this bloom end? Hopefully, this bloom will soon go away so that we can get back to normal. But, history has shown us that when a red tide bloom occurs throughout the summer it will likely result in a fall bloom. In fact the only summer bloom since 1953 that did not result in a fall bloom was in 1960.

Is this our new normal? There is no evidence that this is a trend – next year there may be no blooms.

Additional resources

Current status red tide
The Florida Fish and Wildlife Conservation Commission reports on the current status of Karenia brevis blooms using tables, static maps, and interactive Google Earth maps.http://myfwc.com/redtidestatus